1. Field of the Invention
The present invention relates to an optical transmitter/receiver for transmitting and receiving optical signals between terminal devices of a network in which a plurality of devices are connected by a serial bus, and a network with such optical transmitter/receivers. The serial bus comprises high-speed serial buses standardized according to IEEE (The Institute of Electrical and Electronics Engineers) 1394.
2. Description of the Prior Art
There has in recent years been a demand for the transfer of a large amount of data between terminal devices in view of increased processing capability of computers and in order to meet growing demands for the processing of a large amount of data such as moving image data.
FIG. 1 of the accompanying drawings illustrates a network which employs a serial bus comprising a twisted pair cable with feeder lines. FIG. 2 of the accompanying drawings shows the twisted pair cable with feeder lines in cross section. As shown in FIG. 1, the network has a daisy chain configuration in which some of terminal devices 19a.about.19f serve as repeaters and a branch structure in which three signal lines are connected to the terminal device 19b. The daisy chain configuration is disadvantageous in that when the power supply of the terminal device 19e is turned off, the terminal device 19f is separated from the network and cannot communicate with the other terminal devices. This shortcoming can be alleviated by connecting the terminal devices with a twisted pair cable which comprises transmission lines and feeder lines, so that a physical layer of a terminal device whose main power supply is turned off is supplied with electric energy from another terminal device through the feeder lines for allowing all the terminal devices to communicate with each other.
One twisted pair cable is shown in FIG. 2. As shown in FIG. 2, a twisted pair cable 10 comprises two feeder lines 11a, 11b and two twisted pairs 12a, 12b each insulated by a shield 21. The insulated twisted pairs 12a, 12b and the feeder lines 11a, 11b are insulated together by a shield 22.
In the network shown in FIG. 1, a bias voltage is applied to the twisted pairs at each of the terminal devices and monitored between the terminals for detecting whether the twisted pairs are inserted or pulled out. A detected signal is used as a sign to start initializing the network.
Signals present on a transmission line include two signals, i.e., a signal for bus management (hereinafter referred to as an "arbitration signal") and a data signal, and are time-division-multiplexed for transmission over the transmission line. The transmission line comprises two twisted pairs. The arbitration signal comprises a two-sequence ternary signal using three voltage levels (logic 0, Z, 1), and can transmit a maximum of 9 arbitration signal levels. The data signal comprises a two-sequence binary signal having two voltage levels (logic 0, 1). One of the two sequences of the data signal is a strobe signal generated from a data signal and a clock signal.
The network with the twisted pairs as the transmission line has a problem in that if the transmission rate is increased, the loss over the transmission line is increased, so that the transmission line can be used only for communications over a short distance. Therefore, the area covered by the network is small and limited. Long-distance communications are made possible by using a plurality of repeater terminals, which however increases the cost of the network.
Another approach for long-distance communications is to use optical fibers as the transmission line for an increased transmission range. Conventional optical repeaters which have only a function of repeating electric and optical signals fail to transmit over optical fibers the bias voltage that is applied to the twisted pairs for detecting whether the transmission line is inserted or pulled out. Therefore, when those conventional optical repeaters are used, it is impossible to detect insertion and removal of the transmission line between the terminal devices. In addition, if a duplex optical fiber is employed, then it is necessary to convert the two-sequence ternary and binary signals that are time-division-multiplexed into a one-sequence binary signal.
Arbitration is carried out by causing arbitration signals from opposite terminals to collide with each other and the terminals detecting the voltage level on the cable. If an optical repeater is used, then it is necessary to recognize an arbitrary signal outputted by a terminal based on the voltage level on the cable and to transmit the recognized arbitrary signal as an optical signal. However, no optical transmitter/receiver capable of performing such a function has been available.